1. Technical Field of the Invention
The present invention relates to tuning of a local oscillator, and more particularly, to a method for increasing the speed with which a local oscillator tunes to a predetermined frequency in order to more quickly step through a set of power measurements on a number of neighbor list stations.
2. Description of Related Art
Within a digital mobile station, there is a need to be able to adjust the local oscillator to a predetermined frequency as quickly as possible when stepping through a set of power measurements on neighbor list stations. By quickly stepping through the frequencies on which power measurements are made the mobile station hardware may be turned off for longer periods of time once the power measurements are completed. This saves battery charge and provides longer operating periods for a mobile station operating solely on battery power.
A conventional, prior art mobile station 10 is shown in FIG. 1. The mobile station 10 includes a controller 12 which controls the functions of the mobile station 10. The controller 12 typically includes a central processing unit (CPU) (not shown), memory (not shown), and I/O ports (not shown). The controller 12 processes voice or data signals to and from a transceiver 14. The transceiver 14 converts a voice or data signal from the controller 12 to a radio wave and also detects and demodulates a received radio wave into a voice or data signal. The transceiver 14 is connected to an antenna 16 for transmission and reception of radio waves.
The controller 12 is also connected to a microphone 18, speaker 20 and user interface 22. The microphone 18 includes a dynamic microphone, condenser microphone, or the like, to transduce a user""s voice into an electrical signal. An analog to digital converter (ADC) (not shown) converts the electrical signal into a digital voice signal. The speaker 20 outputs a received voice signal, and typically includes a digital-to-analog converter (DAC) (not shown) and amplifier (not shown). The user interface 22 includes a display, such as an LED or LCD, and a keypad or other controls. A rechargeable battery 24 provides power to the mobile station 10.
A synthesizer 26 is connected to the controller 12 and transceiver 14. The synthesizer 26 generates signals at variable frequencies in response to an input frequency value from the controller 12. The generated signals are communicated to the transceiver 14 for reception or transmission of a channel about that frequency. The synthesizer 26 includes a phase detector 28, loop filter 30, and voltage controlled oscillator (VCO) 32. A reference crystal 34 is connected to the synthesizer 26 and generates a reference signal 35 at a set frequency fr. The controller 12 controls the frequency output of the synthesizer 26 by transmitting a channel data signal 11 to the synthesizer 26 to store a divisor N value in a Divide-by-N Register 36. The controller 12 then transmits a control signal 13 to the synthesizer 26 to latch the divisor N value. In response, a frequency fo of output signal 33 from the VCO 32 is divided by N. The phase detector 28 compares the phases of the output signal 37 of the Divide-by-N Register 36 and the reference signal 35 from reference crystal 34 to generate an error voltage (Ve) signal 29 proportional to the phase difference between the two signals. The loop filter 30 is a low pass filter that filters the voltage error Ve, signal 29 prior to inputting the signal to the VCO 32. The frequency fo of the output signal 33 of the VCO 32 stabilizes to fo=N*fr and is transmitted to the transceiver 14.
The conventional operation of the mobile station 10 of FIG. 1 during a measurement operation of a set of channels is now described. The mobile station 10 receives a measurement order or a neighbor cell list from the radio base station 4 over a traffic or control channel. The controller 12 waits until the next idle state to perform measurements of the channels designated. During the next idle period, the controller 12 determines the number of channels to measure. The controller 12 stores data for the first channel to be measured in the Divide-by-N Register 36 of the synthesizer 26 and transmits a xe2x80x9clatch new channel dataxe2x80x9d signal 13 to the synthesizer 26. In response, the synthesizer 26 generates a VCO output signal 33 at a frequency fo specified by the channel data in the Divide-by-N Register 36. The VCO output signal 33 is communicated to the transceiver 14 which then receives a measurement command 15 from the controller 12. The transceiver 14 measures the signal strength of the first channel and transmits a measured signal strength data signal 17 to the controller 12.
The controller 12 receives the measured signal strength data and decrements the number of channels to measure. The controller 12 determines if additional channels are to be measured. If no more channels are to be measured, the controller 12 again enters an idle period and is ready to transmit or receive on a traffic channel or an active state. Otherwise, the controller 12 loads the channel data for the next channel to be measured into the synthesizer 26. If there is not sufficient time to complete the measurements during the idle period between transmitting or receiving, the mobile station 10 must wait until the next idle period to continue the measurements.
The power measuring operation in a mobile station needs to be performed quickly and efficiently, especially when a mobile station is measuring channels during an idle period, since there is limited time to perform the operation. The process normally taking up the greatest amount of time is the tuning of the local oscillator 32 to the frequency being measured. A need has thus arisen in the industry for a more efficient manner for tuning an oscillator to enable faster measurement of varying sets of channels in a neighbor list by a mobile station.
One solution involves the use of so-called speedup mechanisms within the phase locked loop of the mobile station. These mechanisms push additional charge current into the phase locked loop in an effort to more quickly adjust the frequency of the oscillator. However, this method has an inherent limit on how fast the frequency can change before effects of changing the loop gain cause longer settling transients within the phase locked loop. Thus, an improved method for more quickly tuning a local oscillator signal to a desired frequency in order to more rapidly step through power measurements for neighbor list of mobile stations is needed.
The present invention overcomes the foregoing and other problems with a method and apparatus for more quickly tuning an oscillator to a selected frequency within a mobile station during power measurements. A switch, controller and associated analog to digital and digital to analog converters are located between an output of a phase locked loop and the voltage control input of a local oscillator. The phase locked loop consists of all the parts of the synthesizer and reference crystal less the voltage controlled oscillator. The switch has a first and second positions. In the first position, the output of the phase locked loop is passed directly to the voltage control input of the local oscillator. In the second position, an output from the controller, via a digital to analog converter, is provided to the voltage control input of the local oscillator.
In response to a request to perform power measurements on frequencies for members of a neighbor list within a mobile station, an initial determination is made by the controller as to whether the local oscillator has previously tuned to the requested frequency. If not, the switch is place in the first position such that the voltage control signal applied to the local oscillator comes from the phase locked loop. During this time, the applied voltage control signal from the phase locked loop is converted from analog to digital format and stored in a memory associated with the controller at a location associated with the desired frequency.
If the controller determines that the local oscillator has previously tuned to the requested frequency during power measurements, the controller instructs the switch to the second position and a digital voltage control value associated with the requested frequency is retrieved from the memory of the controller. The retrieved value is applied to a digital to analog converter, and the converted analog voltage control signal is applied to the input of the local oscillator via the switch.